As disclosed in Japanese patent application laid-open No. 53-61412, Japanese patent application laid-open No. 54-89811 and Japanese patent application laid-open No. 55-65269, up to now water soluble dye such as acid dye, direct dye, basic dye, etc. dissolved in glycolic solvent and water has been widely used for the inkjet recording liquid. However, since water soluble dyes with high solubility in water are generally used to ensure stability of the recording liquid, the water resistance of the inkjet recorded materials is generally low and the dye on the recorded material may be easily smudged when water is spilled onto it.
As disclosed in Japanese patent application laid-open No. 56-57862 attempts have been made to improve the inferior water resistance by changing the structure of the dye or preparing recording liquid with high basicity. Also, as disclosed in Japanese patent application laid-open No. 50-49004, Japanese patent application laid-open No. 57-36692 and Japanese patent application laid-open No. 59-20696 and Japanese patent application laid-open No. 59-146889, attempts to improve water resistance by apt use of the reaction between the recording paper and recording liquid have been made.
These methods have made remarkable effects for special recording papers but lack versatility in that they are limited to special recording papers and, quite often, the water-resistant quality of the recorded material is insufficient when recording papers other than those specially prescribed are used.
Oil soluble dye dispersed or dissolved in high boiling point solvents and oil soluble dye dissolved in volatile solvents are also recording liquids with good water resistance but are environmentally unpreferable due to the bad odor and exhaust of the solvents. Also problems such as solvent recovery will be required depending on the location where the apparatus is installed and a large amount of recording is executed.
Therefore, in order to improve the water resistance of the printed material, pigment dispersion type recording liquid where the pigment is dispersed in an aqueous medium is under development.
However, whereas the water resistance of the recorded material has improved by changing the colorant from dye to pigment and finely graining the pigment, it has met with a problem of the inability in obtaining a clear image of recorded materials when printed on plain paper such as acid paper, neutral paper generally used in offices.
This is considered to be due to the fact that when a dye type recording liquid is used, the dye dyes the paper fiber and produces clear image whereas when pigment dispersed type recording liquid is used, the pigments are embedded in between the fibers and cannot conceal the background color of the recording paper. The problem that the clearness of recorded image is impaired has become increasingly prominent with more finely graining the pigment.
In order to improve the affinity of pigments to the recording paper fiber of plain paper, etc., there is a method where aqueous resin is added to the recording liquid to fix the pigments onto the fibers. However, as the addition of aqueous resin sufficient to fix the pigments on the fibers increases the viscosity of the recording liquid, problems in practicality of ink discharge stability arise due to the increase in energy required for ink discharge of the inkjet.